The present invention relates more particularly to connecting together two internally-lined unit pipe elements, and more particularly elements presenting a length of 24 meters (m) or 48 m that are installed in oil fields in deep water, e.g. in depths of 2000 m to 3000 m, or even more, from a laying ship fitted with J-lay towers.
In known manner, the ends of said pipe elements are assembled together end to end by welding, and the internal lining inside said pipe elements comes to an end at a significant distance from each end of said pipe elements, e.g. a distance of 100 millimeters (mm) to 200 mm, so that the heating of the steel wall while the ends are being welded together does not damage said lining. There is then a problem of how to protect against corrosion the non-lined zone extending between the end of the lining of pipe element No. N and the end of the lining in the following pipe element No. N+1.
Patent WO 2006/042925 in the name of the Applicant discloses such an assembly of pipes provided with internal linings. The so-called “swagelining” method of providing the internal lining consists in stretching a circular pipe of flexible material, referred to below as the “liner”, so as to reduce its diameter in order to be capable of being inserted in a pipe by being pulled, the rest diameter of said lining being greater than the inside diameter of said pipe. Another way of inserting such a liner is to deform it in folding so as to obtain a cross-section in the form of a kidney bean that can be inscribed inside a circle of much smaller diameter, thereby allowing it to be inserted merely by being pulled through the steel pipe. At the and of being pulled through, the ends that project considerably naturally return to a substantially circular shape and it is simple to fit a plug thereon. By pressurizing the liner with compressed air, it returns to its circular shape and said liner is pressed firmly against the inside wall of the steel pipe.
In that patent WO 2006/042925, a way of assembling together two pipe elements is described in which a tubular junction sleeve of corrosion-resistant steel or of composite material serves to provide continuity in the protection of the pipe walls over the non-lined ends of said pipe elements. More particularly, a tubular junction sleeve is described that consists in a ferrule of corrosion-resistant alloy in the form of a tubular sleeve having a surface of revolution on the inside with notches at its ends and engaged by force in each of the ends of said pipe elements for assembly so as to create increased leaktightness and continuity for the internal lining at the junction between the two pipe elements.
The terms “liner” or “internal lining” as used herein correspond to an internal covering commonly known as a “liner”.
It is important for the contact between the sleeve and the liner to be leaktight in order to avoid any sea water making contact with the welding zone facing said tubular sleeve.
In the event of water penetrating between the sleeve and the pipe, and when the pipe is a water injection pipe, such direct contact could lead to electrochemical corrosion phenomena of the steel pipe and of the weld, assuming that said mechanical connection between the sleeve and the lining is not necessarily leaktight.
Other, ways of assembling two pipe elements or two strings are described in numerous parents mentioned in WO 2006/042925.
The main drawback of the various methods of assembling strings together lies in the fact that the junction is heterogeneous between a coupling element made of metal and a liner made of thermoplastic material, which coupling is based on a principle of compressing the thermoplastic material of the liner, and, in the long term, the liner can suffer seep that has the consequence of leading to leaks should sealing no longer be ensured between the end of the lining and the end of the sleeve, which goes against the looked-for purpose. Furthermore, that phenomenon is amplified in catenary type bottom-to-surface connection pipes since such pipes are permanently in movement under the effect of swell, wind, and currents acting on the anchored floating support on the surface, thereby creating micromovements in each of the coupling elements, which are generally spaced apart by the length of one string, i.e. generally once every 24 m or 48 m.
In WO 2006/042925, notching of the ends of the lining creates compression that co-operates with notching of the ends of the sleeve in order to compensate for such seep.
In a known tubular junction sleeve associated with a pipe comprising at least two pipe elements made of steel with internal lining made of thermoplastic material that are assembled together end to end, the ends of the two pipe elements being welded together:                said tubular junction sleeve is made of a thermoplastic material, preferably identical to the material of said lining, and it is inserted inside the pipe at the abutting ends of the two pipe elements so that the terminal portions at the ends of said sleeve are at least in part in leaktight contact with the respective terminal portions at the ends of said internal linings of the two pipe elements; and        said tubular junction sleeve presents at each of said terminal portions of the sleeve in said zone of leaktight contact with the terminal portions of said linings a said Joule effect heater wire arranged in a spiral on the outer surface of each said terminal portion at the ends of said sleeve, said leaktight contact zone being a zone of fusion welding together the component materials in mutual contact of each said terminal portion of the sleeve and of each said respective terminal portion of said lining, with said heater wire passing therethrough.        
However a pipe of that type with a tubular junction sleeve having a single strand spiral-wound heater wire as described in FIGS. 3B and 4B of the present description, in which the two ends of the spiral-wound single strand wire lead from opposite ends of the spiral on the inner surface of the sleeve at the two ends of said electrofusion zone with leaktight zones can give rise to problems of loss of sealing representing a risk of sea water coating into contact with the metal welding forming the junction between the two pipe elements, when said pipe is used to convey water. This means also, and above all, that said sleeve runs the risk of deforming and in particular of returning to its initial shape of smaller inside diameter in its central zone facing the metal weld between the ends of the two abutting pipe elements, as shown on the left in FIG. 1, if it had such an initial shape; this is because, as a result of said leak, the zone 8 between the outer surface of the sleeve and the inner surfaces of said pipe elements is then at a pressure that is substantially identical to the pressure that exists inside said sleeve, and given the elasticity of said sleeve it naturally returns to its initial shape. This risk of losing sealing arises for the reasons explained below.
Firstly there is no guarantee of sealing through the thickness of the terminal portion of the sleeve along the pre-drilled channel taken by a non-spiral-wound free end of the heater wire in order to pass through the thickness of said sleeve from the first and second ends of the spiral to the inner surface of the sleeve. Furthermore, the zone of leaktight contact obtained by electrofusion or the fusion-welding zone does not cover all of the contact area between said terminal portions of the sleeve and of said respective internal liners, and in particular fusion does not occur with an acceptable level of reliability in the vicinity of the first and last turns of the spiral since although the supply of heat is well controlled between two adjacent turns, it is not well controlled before the first turn or after the last turn. Thus, in the event of said heater wire covering an outer surface of revolution, in particular a cylindrical surface or a frustoconical surface, the interface between the terminal portions of the sleeve and of said respective linings is not leaktight outside the electrofusion zone, i.e. before the first turn or after the last turn. Thus, the channel for the end of the wire at the first end of the spiral closest to the closest end of the sleeve has access to the non-sealed portion of this interface that exists before said first end of the spiral, and that leads solely to the inner surfaces of the sleeve and of the pipe as a whole. In contrast, the channel for the end of the wire at the second end of the spiral closest to the running portion of the sleeve accesses the portion of this non-sealed interface that exists beyond said second and of the spiral leading solely to the outer surface of the sleeve and capable of giving access to said weld. Under such circumstances, if one of the non-spiral-wound free ends of the wire goes from the second end of the spiral to return to the inner surface of the sleeve, then a fluid under pressure, and in particular water, conveyed by the pipe, in particular with service pressures greater than 5 megapascals (MPa) as is usual, can present a high risk of seeping along said wire through the thickness of the sleeve to said second end and then to said weld via said interface extending between said second end of the spiral and the outer surface of the current portion of the sleeve.
WO 2010/041016 and EP 0 722 824 propose a tubular junction sleeve having a spiral-wound heater wire on its outer surface for which the problem of sealing the metal weld against leaks via the channel or furrow for passing the heater wire through the mass of the sleeve is solved.
In EP 0 722 824, the problem of sealing is solved, by causing the end of the wire beside the electrofusion zone closest to the weld to lead to the outer surface of the sleeve in a central setback, which setback is designed to be plugged by an insulating material after electrofusion and before making the metal weld. Thus, in EP 0 722 824, it is necessary to make the metal weld after electrofusion of the sleeve, and said electrofusion cannot be performed exclusively from inside said sleeve and thus inside said pipe.
In document WO 2010/041016, as shown in FIG. 1, a heater wire is used that is arranged in two spaced-apart helical portions, the furthest-apart ends of the two helical portions being connected together, and the two ends of the heater wire lead to the inner surface of the sleeve beside the gap between the two helical portions. Thus, after electrofusion there is always a leaktight electrofusion zone between the metal weld and the two free ends of the heater wire leading to the inner surface of the sleeve. That embodiment requires the sleeve to be machined and the heater wire to be laid in a manner that is relatively more elaborate and difficult to perform.